Program counter range comparator with equality, greater than, less than and non-equal detection modes

ABSTRACT

An program counter address comparator includes two comparators comparing an input program counter address with respective reference addresses. The comparators produce a match indication on selectable criteria, such as greater than, less than, equal to, not equal to, less than or equal to, and greater than or equal to, and can be selectively chained. Input multiplexers permit selection of either the program counter address bus or a secondary address bus. The reference addresses and control functions are enabled via central processing unit accessible memory mapped registers.

TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of this invention is emulation hardwareparticularly for highly integrated digital signal processing systems.

BACKGROUND OF THE INVENTION

[0002] Advanced wafer lithography and surface-mount packaging technologyare integrating increasingly complex functions at both the silicon andprinted circuit board level of electronic design. Diminished physicalaccess to circuits for test and emulation is an unfortunate consequenceof denser designs and shrinking interconnect pitch. Designed-intestability is needed so the finished product is both controllable andobservable during test and debug. Any manufacturing defect is preferablydetectable during final test before a product is shipped. This basicnecessity is difficult to achieve for complex designs without takingtestability into account in the logic design phase so automatic testequipment can test the product.

[0003] In addition to testing for functionality and for manufacturingdefects, application software development requires a similar level ofsimulation, observability and controllability in the system orsub-system design phase. The emulation phase of design should ensurethat a system of one or more ICs (integrated circuits) functionscorrectly in the end equipment or application when linked with thesystem software. With the increasing use of ICs in the automotiveindustry, telecommunications, defense systems, and life support systems,thorough testing and extensive real-time debug becomes a critical need.

[0004] Functional testing, where the designer generates test vectors toensure conformance to specification, still remains a widely used testmethodology. For very large systems this method proves inadequate inproviding a high level of detectable fault coverage. Automaticallygenerated test patterns are desirable for full testability, andcontrollability and observability. These are key goals that span thefull hierarchy of test from the system level to the transistor level.

[0005] Another problem in large designs is the long time and substantialexpense involved in design for test. It would be desirable to havetestability circuitry, system and methods that are consistent with aconcept of design-for-reusability. In this way, subsequent devices andsystems can have a low marginal design cost for testability, simulationand emulation by reusing the testability, simulation and emulationcircuitry, systems and methods that are implemented in an initialdevice. Without a proactive testability, simulation and emulation plan,a large amount of subsequent design time would be expended on testpattern creation and upgrading.

[0006] Even if a significant investment were made to design a module tobe reusable and to fully create and grade its test patterns, subsequentuse of a module may bury it in application specific logic. This wouldmake its access difficult or impossible. Consequently, it is desirableto avoid this pitfall.

[0007] The advances of IC design are accompanied by decreased internalvisibility and control, reduced fault coverage and reduced ability totoggle states, more test development and verification problems,increased complexity of design simulation and continually increasingcost of CAD (computer aided design) tools. In the board design the sideeffects include decreased register visibility and control, complicateddebug and simulation in design verification, loss of conventionalemulation due to loss of physical access by packaging many circuits inone package, increased routing complexity on the board, increased costsof design tools, mixed-mode packaging, and design for produceability. Inapplication development, some side effects are decreased visibility ofstates, high speed emulation difficulties, scaled time simulation,increased debugging complexity, and increased costs of emulators.Production side effects involve decreased visibility and control,complications in test vectors and models, increased test complexity,mixed-mode packaging, continually increasing costs of automatic testequipment and tighter tolerances.

[0008] Emulation technology utilizing scan based emulation andmultiprocessing debug was introduced more than 10 years ago. In 1988,the change from conventional in circuit emulation to scan basedemulation was motivated by design cycle time pressures and newlyavailable space for on-chip emulation. Design cycle time pressure wascreated by three factors. Higher integration levels, such as increaseduse of on-chip memory, demand more design time. Increasing clock ratesmean that emulation support logic causes increased electricalintrusiveness. More sophisticated packaging causes emulator connectivityissues. Today these same factors, with new twists, are challenging theability of a scan based emulator to deliver the system debug facilitiesneeded by today's complex, higher clock rate, highly integrated designs.The resulting systems are smaller, faster, and cheaper. They have higherperformance and footprints that are increasingly dense. Each of thesepositive system trends adversely affects the observation of systemactivity, the key enabler for rapid system development. The effect iscalled “vanishing visibility.”

[0009]FIG. 1 illustrates the trend in visibility and control over timeand greater system integration. Application developers prefer theoptimum visibility level illustrated in FIG. 1. This optimum visibilitylevel provides visibility and control of all relevant system activity.The steady progression of integration levels and increases in clockrates steadily decrease the actual visibility and control available overtime. These forces create a visibility and control gap, the differencebetween the optimum visibility and control level and the actual levelavailable. Over time, this gap will widen. Application development toolvendors are striving to minimize the gap growth rate. Development toolssoftware and associated hardware components must do more with lessresources and in different ways. Tackling this ease of use challenge isamplified by these forces.

[0010] With today's highly integrated System-On-a-Chip (SOC) technology,the visibility and control gap has widened dramatically over time.Traditional debug options such as logic analyzers and partitionedprototype systems are unable to keep pace with the integration levelsand ever increasing clock rates of today's systems. As integrationlevels increase, system buses connecting numerous subsystem componentsmove on chip, denying traditional logic analyzers access to these buses.With limited or no significant bus visibility, tools like logicanalyzers cannot be used to view system activity or provide the triggermechanisms needed to control the system under development. A loss ofcontrol accompanies this loss in visibility, as it is difficult tocontrol things that are not accessible.

[0011] To combat this trend, system designers have worked to keep thesebuses exposed. Thus the system components were built in a way thatenabled the construction of prototyping systems with exposed buses. Thisapproach is also under siege from the ever-increasing march of systemclock rates. As the central processing unit (CPU) clock rates increase,chip to chip interface speeds are not keeping pace. Developers find thata partitioned system's performance does not keep pace with itsintegrated counterpart, due to interface wait states added to compensatefor lagging chip to chip communication rates. At some point, thisperformance degradation reaches intolerable levels and the partitionedprototype system is no longer a viable debug option. In the current eraproduction devices must serve as the platform for applicationdevelopment.

[0012] Increasing CPU clock rates are also limiting availability ofother simple visibility mechanisms. Since the CPU clock rates can exceedthe maximum I/O state rates, visibility ports exporting information innative form can no longer keep up with the CPU. On-chip subsystems arealso operated at clock rates that are slower than the CPU clock rate.This approach may be used to simplify system design and reduce powerconsumption. These developments mean simple visibility ports can nolonger be counted on to deliver a clear view of CPU activity. Asvisibility and control diminish, the development tools used to developthe application become less productive. The tools also appear harder touse due to the increasing tool complexity required to maintainvisibility and control. The visibility, control, and ease of use issuescreated by systems-on-a-chip tend to lengthen product developmentcycles.

[0013] Even as the integration trends present developers with a toughdebug environment, they also present hope that new approaches to debugproblems will emerge. The increased densities and clock rates thatcreate development cycle time pressures also create opportunities tosolve them. On-chip, debug facilities are more affordable than everbefore. As high speed, high performance chips are increasingly dominatedby very large memory structures, the system cost associated with therandom logic accompanying the CPU and memory subsystems is dropping as apercentage of total system cost. The incremental cost of severalthousand gates is at an all time low. Circuits of this size may in somecases be tucked into a corner of today's chip designs. The incrementalcost per pin in today's high density packages has also dropped. Thismakes it easy to allocate more pins for debug. The combination ofaffordable gates and pins enables the deployment of new, on-chipemulation facilities needed to address the challenges created bysystems-on-a-chip.

[0014] When production devices also serve as the application debugplatform, they must provide sufficient debug capabilities to supporttime to market objectives. Since the debugging requirements vary withdifferent applications, it is highly desirable to be able to adjust theon-chip debug facilities to balance time to market and cost needs. Sincethese on-chip capabilities affect the chip's recurring cost, thescalability of any solution is of primary importance. “Pay only for whatyou need” should be the guiding principle for on-chip tools deployment.In this new paradigm, the system architect may also specify the on-chipdebug facilities along with the remainder of functionality, balancingchip cost constraints and the debug needs of the product developmentteam.

[0015]FIG. 2 illustrates an emulator system 100 including four emulatorcomponents. These four components are: a debugger application program110; a host computer 120; an emulation controller 130; and on-chip debugfacilities 140. FIG. 2 illustrates the connections of these components.Host computer 120 is connected to an emulation controller 130 externalto host 120. Emulation controller 130 is also connected to target system140. The user preferably controls the target application on targetsystem 140 through debugger application program 110.

[0016] Host computer 120 is generally a personal computer. Host computer120 provides access the debug capabilities through emulator controller130. Debugger application program 110 presents the debug capabilities ina user-friendly form via host computer 120. The debug resources areallocated by debug application program 110 on an as needed basis,relieving the user of this burden. Source level debug utilizes the debugresources, hiding their complexity from the user. Debugger applicationprogram 110 together with the on-chip trace and triggering facilitiesprovide a means to select, record, and display chip activity ofinterest. Trace displays are automatically correlated to the source codethat generated the trace log. The emulator provides both the debugcontrol and trace recording function.

[0017] The debug facilities are preferably programmed using standardemulator debug accesses through a JTAG or similar serial debuginterface. Since pins are at a premium, the preferred embodiment of theinvention provides for the sharing of the debug pin pool by trace,trigger, and other debug functions with a small increment in siliconcost. Fixed pin formats may also be supported. When the pin sharingoption is deployed, the debug pin utilization is determined at thebeginning of each debug session before target system 140 is directed torun the application program. This maximizes the trace export bandwidth.Trace bandwidth is maximized by allocating the maximum number of pins totrace.

[0018] The debug capability and building blocks within a system mayvary. Debugger application program 100 therefore establishes theconfiguration at runtime. This approach requires the hardware blocks tomeet a set of constraints dealing with configuration and registerorganization. Other components provide a hardware search capabilitydesigned to locate the blocks and other peripherals in the system memorymap. Debugger application program 110 uses a search facility to locatethe resources. The address where the modules are located and a type IDuniquely identifies each block found. Once the IDs are found, a designdatabase may be used to ascertain the exact configuration and all systeminputs and outputs.

[0019] Host computer 120 generally includes at least 64 Mbytes of memoryand is capable of running Windows 95, SR-2, Windows NT, or laterversions of Windows. Host computer 120 must support one of thecommunications interfaces required by the emulator. These may include:Ethernet 10T and 100T, TCP/IP protocol; Universal Serial Bus (USB);Firewire IEEE 1394; and parallel port such as SPP, EPP and ECP.

[0020] Host computer 120 plays a major role in determining the real-timedata exchange bandwidth. First, the host to emulator communication playsa major role in defining the maximum sustained real-time data exchangebandwidth because emulator controller 130 must empty its receivereal-time data exchange buffers as fast as they are filled. Secondly,host computer 120 originating or receiving the real-time data exchangedata must have sufficient processing capacity or disc bandwidth tosustain the preparation and transmission or processing and storing ofthe received real-time data exchange data. A state of the art personalcomputer with a Firewire communication channel (IEEE 1394) is preferredto obtain the highest real-time data exchange bandwidth. This bandwidthcan be as much as ten times greater performance than other communicationoptions.

[0021] Emulation controller 130 provides a bridge between host computer120 and target system 140. Emulation controller 130 handles all debuginformation passed between debugger application program 110 running onhost computer 120 and a target application executing on target system140. A presently preferred minimum emulator configuration supports allof the following capabilities: real-time emulation; real-time dataexchange; trace; and advanced analysis.

[0022] Emulation controller 130 preferably accesses real-time emulationcapabilities such as execution control, memory, and register access viaa 3, 4, or 5 bit scan based interface. Real-time data exchangecapabilities can be accessed by scan or by using three higher bandwidthreal-time data exchange formats that use direct target to emulatorconnections other than scan. The input and output triggers allow othersystem components to signal the chip with debug events and vice-versa.Bit I/O allows the emulator to stimulate or monitor system inputs andoutputs. Bit I/O can be used to support factory test and other lowbandwidth, non-time-critical emulator/target operations. Extendedoperating modes are used to specify device test and emulation operatingmodes. Emulator controller 130 is partitioned into communication andemulation sections. The communication section supports hostcommunication links while the emulation section interfaces to thetarget, managing target debug functions and the device debug port.Emulation controller 130 communicates with host computer 120 using oneof industry standard communication links outlined earlier herein. Thehost to emulator connection is established with off the shelf cablingtechnology. Host to emulator separation is governed by the standardsapplied to the interface used.

[0023] Emulation controller 130 communicates with the target system 140through a target cable or cables. Debug, trace, triggers, and real-timedata exchange capabilities share the target cable, and in some cases,the same device pins. More than one target cable may be required whenthe target system 140 deploys a trace width that cannot be accommodatedin a single cable. All trace, real-time data exchange, and debugcommunication occurs over this link. Emulator controller 130 preferablyallows for a target to emulator separation of at least two feet. Thisemulation technology is capable of test clock rates up to 50 0 and traceclock rates from 200 to 300 MHZ, or higher. Even though the emulatordesign uses techniques that should relax target system 140 constraints,signaling between emulator controller 130 and target system 140 at theserates requires design diligence. This emulation technology may imposerestrictions on the placement of chip debug pins, board layout, andrequires precise pin timings. On-chip pin macros are provided to assistin meeting timing constraints.

[0024] The on-chip debug facilities offer the developer a rich set ofdevelopment capability in a two tiered, scalable approach. The firsttier delivers functionality utilizing the real-time emulation capabilitybuilt into a CPU's mega-modules. This real-time emulation capability hasfixed functionality and is permanently part of the CPU while the highperformance real-time data exchange, advanced analysis, and tracefunctions are added outside of the core in most cases. The capabilitiesare individually selected for addition to a chip. The addition ofemulation peripherals to the system design creates the second tierfunctionality. A cost-effective library of emulation peripheralscontains the building blocks to create systems and permits theconstruction of advanced analysis, high performance real-time dataexchange, and trace capabilities. In the preferred embodiment fivestandard debug configurations are offered, although customconfigurations are also supported. The specific configurations arecovered later herein.

SUMMARY OF THE INVENTION

[0025] The comparator described here can operate as two individualcomparators each of them comparing the program counter (PC) value of aprocessor against a reference value stored in a register, or it canoperate as a single device that compares the program counter against tworeference values that form a double bounded range.

[0026] The specific resources required for the implementation of thecomparator can vary greatly depending on the requirements andconstraints of a given implementation. In the preferred embodiment, therange comparator formed of two single reference comparators requiresfive user visible registers in addition to the core comparator function.

[0027] Each of the two comparators can produce independent or dependentresults based on selection made via a control register. The comparatorcan generate a match based on one of six comparison criteria: 1) programcounter equal to reference; 2) program counter not equal to reference;3) program counter greater than reference; 4) program counter greaterthan or equal to reference; 5) program counter less than reference; and6) program counter less than or equal to reference.

[0028] The comparator can be programmed via an interface to memorymapped registers. This interface is called the configuration bus (cfgb).

[0029] The comparator control logic includes a lookup table that can beprogrammed in order to add flexibility to the logic behavior of thecomparator.

[0030] The comparator capabilities can be exploited as a tool to be usedby a hardware debug system that will take the comparator result as aninput and will provide a wide range of responses depending on theconfiguration of the debug system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] These and other aspects of this invention are illustrated in thedrawings, in which:

[0032]FIG. 1 illustrates the visibility and control of typicalintegrated circuits as a function of time due to increasing systemintegration;

[0033]FIG. 2 illustrates an emulation system to which this invention isapplicable;

[0034]FIG. 3 illustrates in block diagram form a typical integratedcircuit employing configurable emulation capability; and

[0035]FIG. 4 illustrates in block diagram form two coupled programcounter comparators.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] Processor code developers face many challenges on getting thesoftware to work during the early stages of development. This isspecially true if they are new to the software implementation or theprocessor architecture. The debugging of software is a very complextask. The ability to trace or remain aware of the program counter valueas the software executes is one of the most valuable pieces ofinformation on troubleshooting code problems. Thus a comparator like theone described in this application is a powerful tool on the codedebugging process. Other potential applications not related to debuggingcould include detection of program counter values for triggering systemevent interactions or change in processor operation context.

[0037] The comparator provides the ability to detect the occurrence orabsence of an specific value or ranges of values in the processor'sprogram counter. Having the comparator integrated within a hardwaredebugging system enhances the capability of the system for detecting andresolving problems in the code being executed by the processor.

[0038] The presence of the lookup table and the six different comparisoncriteria modes in the comparator here described enhance its capabilitiesand flexibility in the detection of software problems when debugging isbased of the program counter value.

[0039] Enhancement of the capability and flexibility for detection ofthe occurrence or absence of an specific value or ranges of values inthe processor's program counter.

[0040]FIG. 3 illustrates an example of one on-chip debug architectureembodying target system 140. The architecture uses several moduleclasses to create the debug function. One of these classes is eventdetectors including bus event detectors 210, auxiliary event detectors211 and counters/state machines 213. A second class of modules istrigger generators including trigger builders 220. A third class ofmodules is data acquisition including trace collection 230 andformatting. A fourth class of modules is data export including traceexport 240, and real-time data exchange export 241. Trace export 240 iscontrolled by clock signals from local oscillator 245. Local oscillator245 will be described in detail below. A final class of modules is scanadaptor 250, which interfaces scan input/output to CPU core 201. Finaldata formatting and pin selection occurs in pin manager and pin micros260.

[0041] The size of the debug function and its associated capabilitiesfor any particular embodiment of a system-on-chip may be adjusted byeither deleting complete functions or limiting the number of eventdetectors and trigger builders deployed. Additionally, the tracefunction can be incrementally increased from program counter trace onlyto program counter and data trace along with ASIC and CPU generateddata. The real-time data exchange function may also be optionallydeployed. The ability to customize on-chip tools changes the applicationdevelopment paradigm. Historically, all chip designs with a given CPUcore were limited to a fixed set of debug capability. Now, an optimizeddebug capability is available for each chip design. This paradigm changegives system architects the tools needed to manage product developmentrisk at an affordable cost. Note that the same CPU core may be used withdiffering peripherals with differing pin outs to embody differingsystem-on-chip products. These differing embodiments may requirediffering debug and emulation resources. The modularity of thisinvention permits each such embodiment to include only the necessarydebug and emulation resources for the particular system-on-chipapplication.

[0042] The real-time emulation debug infrastructure component is used totackle basic debug and instrumentation operations related to applicationdevelopment. It contains all execution control and register visibilitycapabilities and a minimal set of real-time data exchange and analysissuch as breakpoint and watchpoint capabilities. These debug operationsuse on-chip hardware facilities to control the execution of theapplication and gain access to registers and memory. Some of the debugoperations which may be supported by real-time emulation are: setting asoftware breakpoint and observing the machine state at that point;single step code advance to observe exact instruction by instructiondecision making; detecting a spurious write to a known memory location;and viewing and changing memory and peripheral registers.

[0043] Real-time emulation facilities are incorporated into a CPUmega-module and are woven into the fabric of CPU core 201. This assuresdesigns using CPU core 201 have sufficient debug facilities to supportdebugger application program 110 baseline debug, instrumentation, anddata transfer capabilities. Each CPU core 201 incorporates a baselineset of emulation capabilities. These capabilities include but are notlimited to: execution control such as run, single instruction step, haltand free run; displaying and modifying registers and memory; breakpointsincluding software and minimal hardware program breakpoints; andwatchpoints including minimal hardware data breakpoints.

[0044]FIG. 4 illustrates comparator 400 including two individual programcounter comparators that integrate range comparison. Comparator 400 ispreferably a part of bus event detectors 210. The first comparator is atthe top of FIG. 4 and the second comparator is at the bottom. Eachaddress comparator has 4 main components: a program counter samplingmechanism (multiplexers 411 and 421); configuration resources (busselect register 402, comparator control registers 403 and 405, andaddress reference registers 404 and 406); a program counter magnitudecomparator (memory address magnitude comparators 412 and 422); and aprogram counter compare control block (413 and 423).

[0045]FIG. 4 illustrates two multiplexers 411 and 421 serving as theprogram counter sampling mechanism. Multiplexers 411 and 421 select thedesired bus signal from the program counter bus and other busses. Theuser controls this bus selection of the two comparators by writing tothe bus select register 402.

[0046]FIG. 4 illustrates 5 registers which are part of the comparatorconfiguration. Bus select register 402, comparator control registers 403and 405 and address reference registers 404 and 406 can be accessed likeother configurable hardware resource in the processor via a memoryinterface called a configuration bus interface (cfgb). This memoryinterface includes configuration interface control 401 which receives aconfiguration control signal (cfgb_ctl). Configuration data (cfgb_data)and a configuration clock (cfgb_clk) are supplied to bus select register402, comparator control registers 403 and 405 and address referenceregisters 404 and 406. Upon receipt of a register write request and theidentity of the register via cfbg_ctl, configuration interface control401 selects which register stores the data on the configuration databus. The selected register stores the data on the configuration data busupon the next configuration clock signal.

[0047] Data stored in bus select register 402 defines via multiplexers411 and 421 which input bus is compared against the respective referencevalues. Multiplexer 411 selects either the program counter bus or theother bus for supply to program counter magnitude comparator 412. Theselection of multiplexer 411 is controlled by bus select register 402via port_select_(—)0 signal. Address reference register 404 supplies areference address to program counter magnitude comparator 413 for thecomparison. Multiplexer 421 similarly selects the bus for programcounter magnitude comparator 422 under control of bus select register402 via port_select_(—)1 signal. Address reference register 406 storesthe reference address for program counter magnitude comparator 422 undercontrol of bus select register 402.

[0048] Program counter magnitude comparators 412 and 422 perform theactual comparison between the reference value and the selected programcounter bus. Memory address magnitude comparators 412 and 422 eachgenerate an equal signal (EQ) if the program counter equals thereference address, a greater than signal (GT) if the program counter isgreater than the reference address or a less than signal (LT) if theprogram counter is less than the reference address. Program countermagnitude comparator 412 receives a configuration and control signalconfig_&_control_(—)0 from comparator control register 403. Programcounter magnitude comparator 422 receives a similar configuration andcontrol signal config_&_control_(—)1 from comparator control register405.

[0049] Outputs GT, LT, EQ from the memory address magnitude comparators412 and 422 are supplied to respective memory access compare controlblocks 413 and 423. Program counter compare control blocks 413 and 423generate corresponding pc_event outputs depending on these inputsignals. Program counter compare control blocks 413 and 423 takes the 3result signals (GT, LT, EQ) from corresponding program counter magnitudecomparators 412 and 422 and expand them into six comparison modeselections. These are: greater than; less than; equal to; not equal to;less than or equal to; and greater than or equal to. Each of the twoprogram counter comparators can be programmed in one of the 6 comparisonmodes. Program counter comparison control block 413 supplieslocal_event_(—)0 signal to program counter comparison control block 423.Program counter comparison control block 423 supplies labeledlocal_event_(—)1 signal to program counter comparison control block 413.

[0050] Each program counter comparison control block 415 and 425receives 4 entry lookup table from the corresponding comparator controlregister 403 and 405 via respective signals look_up_table_(—)0 andlook_up_table_(—)1. The table look up signals enable the final outputspc_event_(—)0 and pc_event_(—)1 to depend upon the local_event signalfrom the other program counter comparison control block. This dependencyenables address range comparisons with each comparator testing one forthe limit values.

What is claimed is:
 1. A program counter address comparator comprising:a first programmable reference address register storing a firstreference address; a first comparator receiving said first referenceaddress from said first programmable reference address register and aprogram counter address input for generating a greater than output, aless than output and an equal to output depending upon the relationshipbetween said first reference address and the program counter address; asecond programmable reference address register storing a secondreference address; a second comparator receiving said second referenceaddress from said second programmable reference address register and theprogram counter address for generating a greater than, less than orequal to output depending upon the relationship between said secondreference address and the program counter address; first and secondprogram counter compare control units connected to respective first andsecond comparators, each of said first and second program countercompare control units generating a corresponding first and second localevent signal selectively dependent upon said greater than output, saidless than output and said equal to output of said correspondingcomparator and a corresponding first and second program counter compareevent signal selectively dependent upon said local signal of the otherprogram counter compare control unit.
 2. The program counter addresscomparator of claim 1, further comprising: a program counter addressbus; a secondary address bus; a first multiplexer having a first inputreceiving said program counter address bus, a second input receivingsaid secondary address bus, a control input and an output connected tosaid address input of said first comparator; a second multiplexer havinga first input receiving said program counter address bus, a second inputreceiving said secondary address bus, a control and an output connectedto said address input of said second comparator; and a programmable busselection register connected to said control input of said first andsecond multiplexers, said programmable bus selection registercontrolling said first and second multiplexers independently to selecteither said program counter address bus or said secondary address busfor supply to said address input of said first and second comparators.3. The program counter address comparator of claim 2, wherein: saidprogrammable bus selection register is programmable via a centralprocessing unit memory mapped register write command.
 4. The programcounter address comparator of claim 1, further comprising: a firstprogrammable comparison control register connected to said first programcounter compare control unit for controlling said first program countercompare control unit to generate said first local signal if said addressinput is a selected one of (1) greater than, (2) less than, (3) equalto, (4) not equal to, (5) less than or equal to, and (6) greater than orequal to said first reference address and selecting whether said firstprogram counter compare event signal is dependent upon or independent ofsaid second local signal; and a second programmable comparison controlregister connected to said second program counter compare control unitfor controlling said second program counter compare control unit togenerate said second local signal if said address input is a selectedone of (1) greater than, (2) less than, (3) equal to, (4) not equal to,(5) less than or equal to, and (6) greater than or equal to said secondreference address and selecting whether said second program countercompare event signal is dependent upon or independent of said firstlocal signal.
 5. The program counter address comparator of claim 4,wherein: said first and second programmable comparison control registersare programmable via corresponding central processing unit memory mappedregister write commands.
 6. The program counter address comparator ofclaim 1, wherein: said first and second programmable address referenceregisters are programmable via corresponding central processing unitmemory mapped register write commands.